Read equalizer for a data storage system

ABSTRACT

A device for scanning a track on a record carrier for reading information has a head for generating a read signal. Marks in the track have a number of different shapes for representing the information. The device processes the read signal by a combination of a linear equalizer ( 81 ) and a non-linear equalizer ( 89 ). The linear equalizer ( 81 ) is arranged for equalizing based on a mark having a single predefined shape, and the non-linear equalizer ( 89 ) is arranged for reducing inter symbol interference in the read signal for marks having a different shape then said predefined shape. The inter symbol interference remaining in the read signal is effectively reduced by the non-linear equalizer ( 89 ) because it is optimized based on the fact that said linear equalizer ( 81 ) is optimized on said single predefined shape.

The invention relates to a device for scanning a track on a recordcarrier for reading information, the device comprising a head forgenerating a read signal via a beam of radiation for scanning marks inthe track, the marks having a number of different shapes forrepresenting the information.

The invention further relates to a method of equalizing a read signalduring reading information in a track on a record carrier, the methodcomprising receiving a read signal generated by marks in the track, themarks having a number of different shapes for representing theinformation.

The record carrier may be of a recordable type and has a track forrecording information, e.g. a spiral shaped track on a disc shapedcarrier. For scanning the track an optical head is positioned at thetrack by a positioning unit. The head has a laser and optical elementsfor generating a beam of radiation for reading marks. The marks arephysical patterns that represent the information and are opticallydetectable. A device and method for equalizing a read signal from such arecord carrier are known from EP 0585095. A reproduction equalizercomprises a linear equalizer for linearly equalizing a source signalread from a recording medium, and a nonlinear cancellation means forcanceling inter symbol interference (ISI) contained in the reproductionsignal. The nonlinear cancellation means includes a lookup table storingthe ISI data, a circuit for generating an address for reading out theISI data from the lookup table, and a circuit for subtracting the readISI data from the equalized source signal. The device has calculationmeans for automatically calculating and/or updating the ISI data held inthe nonlinear cancellation means on the basis of the equalized sourcesignal. At an initial stage, the calculation means calculates the ISIdata, and in a normal operation mode, the calculated ISI data is used. Aproblem is that the known equalization system is not able to reduceinter symbol interference sufficiently in high density recording.

It is an object of the invention to provide a reading device andcorresponding method for effectively repressing inter symbolinterference.

For this purpose, the device as described in the opening paragraph hasread means for processing the read signal the read means comprising acombination of a linear equalizer and a non-linear equalizer forequalizing the read signal, the linear equalizer being arranged forequalizing based on a mark having a single predefined shape, and thenon-linear equalizer being arranged for reducing inter symbolinterference in the read signal for marks having a different shape thensaid predefined shape, the inter symbol interference remaining in theread signal due to said linear equalizer being based on said singlepredefined shape.

The method as described in the opening paragraph comprises processingthe read signal by a combination of linear equalization and a non-linearequalization for equalizing the read signal, the linear equalizationbeing arranged for equalizing based on a mark having a single predefinedshape, and the non-linear equalization being arranged for reducing intersymbol interference in the read signal for marks having a differentshape than said predefined shape, the inter symbol interferenceremaining in the read signal due to said linear equalizer being based onsaid single predefined shape.

The effect of the measures is that the linear equalization is based on apredefined selection of one of the mark shapes to be distinguished, andthe non-linear equalization is adapted to reduce the inter symbolinterference for marks having different shapes based on the fact thatthe linear equalization is optimized for said first shape.

The invention is also based on the following recognition. In recentoptical recording systems multi level codes are used. Multi level codesrequire read signals at different signal levels from a single mark, andthe marks written on the recording medium are often thought of asdifferent levels of grey. The grey levels correspond to the levels ofthe read signal. However, physically, grey cannot be written due to thenature of the recording medium, e.g. phase change material is either ina crystalline or amorphous state, magnetization is either up or down inmagnetic system, etc. The inventors have seen that information inmultilevel recording is contained in the shape of the marks rather thanin the reflectivity. In particular, the information is contained in thelength of marks resulting in different read signal levels. Because thelength of the mark is varied for achieving the required signal level ata read-out time, the read signal at other relevant moments, i.e. at theread-out times of preceding and succeeding symbols, is also influenced,by so called inter symbol interference. Equalization is applied in thereceiver to restore the required signal levels and reduce inter symbolinterference. First the inventors have optimized the linear equalizerfor a selected one of the expected read signal shapes. Secondly theyhave determined residual inter symbol interference based on a newdescriptive channel model, which takes into account said optimizedlinear equalizer being used on a read signal that actually is generatedby marks of different shapes. The non-linear equalizer is optimizedbased on the channel model knowing that the linear equalizer isoptimized on a predefined one of a multitude of different mark shapes.

In an embodiment of the device the read means are for processing theread signal for generating a corresponding number of different levels ofa processed read signal at a read-out time for said number of differentshapes. The equalizer function is particularly suitable for restoringthe read signal in a multilevel storage system. However, it is to benoted that the equalizer may also be used in a different read-outsystem, e.g. for optimizing zero crossings in a binary read-out signal.

In an embodiment of the device said number of different shapes compriseslonger and shorter shapes, and the linear equalizer is arranged forequalizing based on a mark having a longer shape. The inventors haveseen that when the linear equalizer is optimized for a longer shape, theresidual inter symbol interference for the shorter shapes is less. Thishas the advantage that the inter symbol interference can be reducedfurther by the non-linear equalizer.

In an embodiment of the device the linear equalizer is arranged forequalizing based on a mark having the longest shape of said differentshapes. In a practical embodiment the longest shape proved to be thechoice having the least residual inter-symbol interference.

Further embodiments are given in the dependent claims.

These and other aspects of the invention will be apparent from andelucidated further with reference to the embodiments described by way ofexample in the following description and with reference to theaccompanying drawings, in which

FIG. 1 shows diagrammatically an optical recording process,

FIG. 2 shows a scanning device,

FIG. 3 shows a model for the channel of a multilevel storage system,

FIG. 4 shows pulses after equalization,

FIG. 4 a shows pulses using an equalizer optimized for a minimum lengthpulse,

FIG. 4 b shows pulses using an equalizer optimized for a medium lengthpulse,

FIG. 4 c shows pulses using an equalizer optimized for a maximum lengthpulse,

FIG. 5 shows inter symbol interference values dependent on theequalizer,

FIG. 6 shows read signal equalization,

FIG. 7 shows a linear equalizer,

FIG. 8 shows a linear equalizer and adaptation of the coefficients,

FIG. 9 shows an alternative circuit for read signal equalization,

FIG. 10 shows correction values for an ISI calculator, and

FIG. 11 shows correction values for a linearizer.

Corresponding elements in different Figures have identical referencenumerals.

FIG. 1 shows diagrammatically an optical recording process. Relevantelements of a recording device are shown comprising a turntable 1 and adrive motor 2 for rotating a disc shaped record carrier 4 about an axis3 in a direction indicated by an arrow 5. The record carrier has a track11 for recording marks 8, the track being located by a servo pattern forgenerating servo tracking signals for positioning an optical headopposite the track. The servo pattern may for example be a shallowwobbled groove, usually called a pre-groove, and/or a pattern ofindentations, usually called pre-pits or servo pits. The record carrier4 comprises a radiation-sensitive recording layer which upon exposure toradiation of sufficiently high intensity is subjected to an opticallydetectable change, such as for example a change in reflectivity, forforming marks 8 constituting a recorded pattern representinginformation. In the recorded pattern the marks have a specific shape,which represent the information. The representation may be according toa modulation scheme usually called channel code.

The radiation-sensitive layer may consist of material such as aradiation sensitive dye or a phase-change material, whose structure canbe changed from amorphous to crystalline or vice versa under theinfluence of radiation. An optical write head 6 is arranged opposite thetrack of the (rotating) record carrier. The optical write head 6comprises a radiation source, for example a solid-state laser, forgenerating a write beam 13. The intensity I of the write beam 13 ismodulated in conformity with a control signal in a customary manner. Theintensity of the write beam 13 varies between a write intensity, whichis adequate to bring about detectable changes in the optical propertiesof the radiation-sensitive record carrier for forming marks andintermediate areas in between the marks further called space. In a writesystem a low (or zero) intensity, which does not bring about anydetectable changes, may be used for creating spaces. High densityrewriting systems using phase change material are usually based on adirect overwrite (DOW) writing. Therefore when a space is to be written,some write pulse is required to erase possible previous data on thedisc. Usually, a melt pulse (high power) is given, followed by a lowerlevel for a particular period to obtain (partial) regrowth of acrystalline area into the previously molten area. The marks may be inany optically readable form, e.g. in the form of areas with a reflectioncoefficient different from their surroundings, obtained when recordingin materials such as dye, alloy or phase change material or in the formof areas with a direction of magnetization different from theirsurroundings, obtained when recording in magneto-optical material.

For reading the recording layer is scanned with a beam 13 whoseintensity is at a reading level of a constant intensity which is lowenough to preclude a detectable change in optical properties. Duringscanning the read beam reflected from the record carrier is modulated inconformity with the information pattern being scanned. The modulation ofthe read beam can be detected in a customary manner by means of aradiation-sensitive detector which generates a read signal which isindicative of the beam modulation.

FIG. 2 shows a scanning device for writing and/or reading information ona record carrier 11. The record carrier may be of a read-only type, e.g.manufactured by pressing like a CD or DVD-ROM, or the record carrier maybe of a type which is writable or re-writable, for example a recordableDVD or BD (Blu-ray Disc). The device is provided with scanning means forscanning the track on the record carrier which means include a driveunit 21 for rotating the record carrier 11, a scanning unit 22comprising an optical head and additional circuitry, a positioning unit25 for coarsely positioning the optical head in the radial direction onthe track, and a control unit 20. The optical head comprises an opticalsystem of a known type for generating a radiation beam 24 guided throughoptical elements focused to a radiation spot 23 on a track of theinformation layer of the record carrier. The optical head and additionalcircuits constitute a scanning unit for generating signals detected fromthe radiation beam. The radiation beam 24 is generated by a radiationsource, e.g. a laser diode. The head further comprises (not shown) afocusing actuator for moving the focus of the radiation beam 24 alongthe optical axis of said beam and a tracking actuator for finepositioning of the spot 23 in a radial direction on the center of thetrack. The tracking actuator may comprise coils for radially moving anoptical element or may alternatively be arranged for changing the angleof a reflecting element.

It is noted that FIG. 2 shows a scanning device for writing and readinginformation. Alternatively a playback only device may contain only thereading elements described below. For writing information the radiationis controlled to create optically detectable marks in the recordinglayer. For reading the radiation reflected by the information layer isdetected by a detector of a usual type, e.g. a four-quadrant diode, inthe optical head for generating a read signal and further detectorsignals including a tracking error and a focusing error signal forcontrolling said tracking and focusing actuators. The read signal isprocessed by read processing unit 30 including an equalizer according tothe invention, and a demodulator, deformatter and output unit of a usualtype to retrieve the information. Hence elements for reading informationinclude the drive unit 21, the optical head, the positioning unit 25 andthe read processing unit 30. The device comprises write processing meansfor processing the input information to generate a write signal to drivethe optical head, which means comprise an input unit 27, and a formatter28 and a laser power unit 29. The control unit 20 controls the recordingand retrieving of information and may be arranged for receiving commandsfrom a user or from a host computer. The control unit 20 is connectedvia control lines 26, e.g. a system bus, to said input unit 27,formatter 28 and laser power unit 29, to the read processing unit 30,and to the drive unit 21, and the positioning unit 25. The control unit20 comprises control circuitry, for example a microprocessor, a programmemory and control gates, for performing the writing and/or readingfunctions. The control unit 20 may also be implemented as a statemachine in logic circuits.

The control unit 20 is connected via control lines 26, e.g. a systembus, to said input unit 27, formatter 28 and laser power unit 29, to theread processing unit 30, and to the drive unit 21, and the positioningunit 25. The control unit 20 comprises control circuitry, for example amicroprocessor, a program memory and control gates. The control unit 20may also be implemented as a state machine in logic circuits.

In an embodiment the recording device is a storage system only, e.g. anoptical disc drive for use in a computer. The control unit 20 isarranged to communicate with a processing unit in the host computersystem via a standardized interface. Digital data is interfaced to theformatter 28 and the read processing unit 30 directly.

In an embodiment the device is arranged as a stand alone unit, forexample a video recording or playback apparatus for consumer use. Thecontrol unit 20, or an additional host control unit included in thedevice, is arranged to be controlled directly by the user, and toperform the functions of the file management system. The device includesapplication data processing, e.g. audio and/or video processingcircuits. User information is presented on the input unit 27, which maycomprise compression means for input signals such as analog audio and/orvideo, or digital uncompressed audio/video. Suitable compression meansare for example described for audio in WO 98/16014-A1 (PHN 16452), andfor video in the MPEG2 standard. The input unit 27 processes the audioand/or video to units of information, which are passed to the formatter28. The read processing unit 30 may comprise suitable audio and/or videodecoding units.

The formatter 28 is for adding control data and formatting and encodingthe data according to the recording format, e.g. by adding errorcorrection codes (ECC), interleaving and channel coding. Further theformatter 28 comprises synchronizing means for including synchronizingpatterns in the modulated signal. The formatted units comprise addressinformation and are written to corresponding addressable locations onthe record carrier under the control of control unit 20. The formatteddata from the output of the formatter 28 is passed to the laser powerunit 29.

The laser power unit 29 receives the formatted data indicating the marksto be written and generates a laser power control signal which drivesthe radiation source in the optical head. For multilevel recordingdifferent marks are used to generate different levels of the read-outsignal during read-out at a specific read-out time. The track issubdivided in cells of a constant length, and each cell contains a markrepresenting one of a number of signal levels. Traditionally the marksare considered as grey. However, due to the nature of the physicalphenomena used to form the marks, grey is not the physical constitutionof the mark. Actually the read signal level of traditional multilevelsystems is generated by different shapes of the marks, in particular thelength. The laser power unit 29 is arranged for generating a powerpattern for accurately writing marks of a preferred shape. The differentlengths of a mark are not detected as such, but as different levels ofthe read signal value of a symbol in a cell, because the size of aradiation spot for detecting the contents of a cell is about the size ofthe cell itself. In other words, the size of the symbol (the cell) isselected as small as possible with respect to the detection system. Inpractice the radiation spot will also detect some of the contents of theneighboring cells, which causes inter symbol interference (ISI). LinearISI can be compensated by linear equalization, provided that the Nyquistrequirement is met. This requirement says that the symbol rate should beless than twice the bandwidth of the system. In our case, the symbolrate is f_(symbol) and the bandwidth is the optical cutoff being fc=2NA/lambda, so we find that ISI can be fully eliminated (i.e. fullresponse system) provided that f_(symbol)<4 NA/lambda). Non-linear ISIoccurs in practical high-density systems.

For understanding the inter symbol interference a model for the channelof writing and reading of marks is discussed now. First, the residualISI due to non-linear behavior of a pulse width (or duration) modulated(PWM) system is calculated. It is shown that the residual ISI is notnegligible. The effects of ISI can be reduced by measures during writing(predistortion in the write channel, not discussed here), but also atread back, by equalization. The equalizer according to the invention isbased on the following model.

FIG. 3 shows a model for the channel of a multilevel storage system. Achannel 51 is provided with input symbols represented by a[k], which areconverted to a discrete-time waveform by passing them through a pulsemodulator 52. The pulse modulator 52 is described by a Fourier pairc_(p)(t)⇄C_(p)(f). In case of amplitude modulation, there is only onepulse shape used, which is modulated in amplitude by the a_(k). In caseof pulse width modulation, different pulses of different duration areused, dependent on the symbols a_(k) to be transmitted. The pulse isgiven by the Fourier pair:${c_{p}(t)} = {\left. {\Pi\left( \frac{t}{D} \right)}\longleftrightarrow{C_{p}(f)} \right. = \frac{\sin\left( {\pi\quad{fD}} \right)}{\pi\quad f}}$

The block function II used above is defined according to:${\Pi\left( \frac{t}{D} \right)} = \left\{ \begin{matrix}1 & {{{for} - \frac{D}{2}} \leq t \leq \frac{D}{2}} \\0 & {elsewhere}\end{matrix} \right.$

In pulse width modulation, the duration D depends on the symbol to betransmitted, e.g. according to ${D = {\frac{p}{M}T}},$in which M is the alphabet size, $T = \frac{1}{f_{symbol}}$is the symbol time, and p is the pulse index. The optical channel isspecified by its modulation transfer function (MTF):${{MTF}(f)} = \begin{Bmatrix}{{{{\frac{2}{\pi}{\arccos\left( \frac{f}{f_{c}} \right)}} - {\frac{2{f}}{\pi\quad f_{c}}\sqrt{1 - \left( \frac{f}{f_{c}} \right)^{2}}{for}\quad{f}}} \leq f_{c}}\quad} \\{{0\quad{for}\quad{f}} > f_{c}}\end{Bmatrix}$in which $f_{c} = \frac{2{NA}}{\lambda}$is the optical cut-off of the channel (NA being numerical aperture ofthe lens and λ being the wavelength). The equalizer EQ is chosen such asto obtain an ISI-free (known as full-response, or FR) system. From themodel as shown in FIG. 3 it is clear that the FR equalizer is pulsemodulator dependent as the total response has to satisfy RCβ(f). Theindex e is used to emphasize that the equalizer belongs to pulse e. Asthe pulse is not known in advance, it is not possible to use thecorresponding equalizer in the receiver. Consequently, a pulse widthmodulation system cannot be FR. Instead, the system will be non-linearand show residual ISI. By proper compensation, non-linearity and ISI canbe made small. Compensation may be applied via precompensation in thewrite channel. However, compensation may also be done in the readchannel, which is advantageous when it is not possible to accuratelycontrol the write channel (read only discs like ROM and R, or writtendiscs like RW after ageing). Initially assuming a linear system, thechannel is made ISI-free by using a transfer function showing vestigialsymmetry around half the symbol rate (according to Nyquist). In thesequel, the so-called raised cosine response, or RC response for short,a commonly function for this purpose, is given by:${{RC}_{\beta}(f)} = \left\{ \begin{matrix}\frac{1}{f_{symbol}} & {{{for}\quad 0} \leq {f} \leq {\frac{1 - \beta}{2}f_{symbol}}} \\{\frac{1}{2f_{symbol}}\left\{ {1 - {\sin\left\lbrack {\frac{\pi}{\beta}\left( {\frac{f}{f_{\quad{symbol}}} - \frac{1}{2}} \right)} \right\rbrack}} \right\}} & {{{for}\quad\frac{1 - \beta}{2}f_{symbol}} \leq {f} \leq {\frac{1 + \beta}{2}f_{symbol}}} \\0 & {{{for}\quad{f}} \geq {\frac{1 + \beta}{2}{f_{symbol}\quad.\quad.\quad.}}}\end{matrix} \right.$

The parameter β determines the excess bandwidth (0≦β≦1, β=0corresponding to no excess bandwidth, i.e. sinc response channel, andβ=1 corresponding to 100% excess bandwidth). Now the cut-off of theRC-function is put at the MTF cut-off (one may do another choice butthis would mean throwing away some of the HF part of the MTF).Consequently, β is not longer an independent parameter, but ratherdirectly coupled to the density on disc. It is given by:$\beta = \frac{{2f_{c}} - f_{symbol}}{f_{symbol}}$

Because β is no longer an independent parameter, it is dropped from thenotation in those cases where the above value is used. Substitutionyields: ${{RC}(f)} = \left\{ \begin{matrix}\frac{1}{f_{symbol}} & {{{for}\quad 0} \leq {f} \leq {f_{symbol} - f_{c}}} \\{\frac{1}{2f_{symbol}}\left\{ {1 - {\sin\left\lbrack {\frac{\pi\quad f_{symbol}}{{2f_{c}} - f_{symbol}}\left( {\frac{f}{f_{\quad{symbol}}} - \frac{1}{2}} \right)} \right\rbrack}} \right\}} & {{{{for}\quad f_{symbol}} - f_{c}} \leq {f} \leq f_{c}} \\0 & {{{for}\quad{f}} \geq {f_{c}\quad.\quad.\quad.}}\end{matrix} \right.$

The equalizer yielding ISI-free response for pulse p is given by:${{EQ}_{p}(f)} = \frac{{RC}_{\beta}(f)}{{C_{p}(f)}{{MTF}(f)}}$

If we now apply a different pulse, i.e. a pulse for which the equalizerwas not made ISI free, residual ISI will result. Suppose the equalizerwas made ISI-free for pulse e, while pulse p is applied, then the outputpulse response function can be written as: $\begin{matrix}{{y(t)} = {\mathcal{F}^{- 1}\left\{ {{C_{p}(f)}{{MTF}(f)}{{EQ}_{e}(f)}} \right\}}} \\{= {\mathcal{F}^{- 1}\left\{ {\frac{{RC}_{\beta}(f)}{C_{e}(f)}{C_{p}(f)}} \right\}}}\end{matrix}$and this result suffers from ISI for p≠e.

FIG. 4 shows pulses after equalization. The pulses are plotted to get animpression of the type of non-linearity and ISI. For this purpose, amultilevel system was taken using M=8.

FIG. 4 a shows pulses using an equalizer optimized for a minimum lengthpulse. The equalizer is optimized for p=1 using the above formulas. Thepulse response y(t) for 8 different pulses is shown, the y-axis beingthe nominal read-out time 61. The signal values at a distance T are theresidual ISI values at the read-out time of the neighboring cells: thenext neighbor 62 and the second succeeding neighbor 63. The nominalmaximum signal level 64 is indicated on the y-axis and corresponds tolevel=8. It can be seen that the pulse response 66 for level=1 has thenominal value of 1 at the y-axis due to the equalizer being determinedfor that pulse. The pulse response 65 for level=8 deviates substantiallyfrom the maximum level 64.

FIG. 4 b shows pulses using an equalizer optimized for a medium lengthpulse. The equalizer is optimized for p=4.5 using the above formulas.The pulse response y(t) for 8 different pulses is shown as in FIG. 6 a.It can be seen that the pulse response 68 for level=1 has the nominalvalue of more than 1 at the y-axis due to the equalizer not beingdetermined for that pulse. The pulse response 67 for level=8 deviatesfrom the maximum level 64, but less than in FIG. 6 a.

FIG. 4 c shows pulses using an equalizer optimized for a maximum lengthpulse. The equalizer is optimized for p=8 using the above formulas. Thepulse response y(t) for 8 different pulses is shown as in FIG. 6 a. Itcan be seen that the pulse response 70 for level=1 has the nominal valueof substantially more than 1 at the y-axis due to the equalizer notbeing determined for that pulse. The pulse response 69 for level=8 nowexactly is at the maximum level 64.

FIG. 5 shows inter symbol interference values dependent on theequalizer. The table gives ISI values for three equalizers, optimizedfor e=1, e=4.5 and e=8. It is noted that the values in the table of FIG.7 correspond to the pulse responses drawn in FIG. 6. The table shows foreach equalizer the signal values at the nominal read-out time (n=0), andthe ISI values at the next three neighbors (n=1,2,3) for eight differentsignal levels (pulse lengths p=1 tot p=8).

It is noted that the model only describes non-linear effects due toequalization of length modulated pulses. There are also other non lineareffects, e.g. the read out of optical discs is intrinsically non-linear.However, as the effect under investigation is quite severe, the currentchannel model by means of a linear MTF is a practical tool. Further, themodel assumes that marks are only modulated in length, and not inamplitude or shape. Measurements confirm that length modulation is themain effect in the current high density media (e.g. using fast coolingfast growth phase change material).

From the above it is concluded that a combination of linearization andISI compensation are required. Hence first a linear equalizer isoptimized for a single length of the marks, as discussed with referenceto FIG. 7. Secondly a non-linear equalizer is optimized taking intoaccount the optimized linear equalizer based on the model, e.g. as shownin FIG. 5. Non-linearity as well as ISI appear not to be very dependenton density, though the effect tends to decrease slightly with density.The non-linear and ISI effects mainly are a property of the pulse lengthmodulation system, and can be successfully compensated by theequalization as proposed.

For read signal equalization the proposed compensation span is at leastnearest neighbor (3 taps), but may be one more neighbor (5-taps) mayfurther improve system performance. If the ISI is not too severe, anapproximation of the ISI is made from a single sample (in which furtherISI is neglected), followed by subtraction of this approximated valuefrom the neighboring signal samples. For a system having more severe ISIthe neighboring samples may be also included for calculating ISIcorrection values. The correction values may be calculated or a lookuptable may be included for providing table lookup or a non-linearfunction in a finite impulse response (FIR) equalizer. The idea isimplemented in the non-linear equalizers shown in FIGS. 6 and 9.

FIG. 6 shows read signal equalization by a combination of a linearequalizer and a non-linear equalizer. A read signal is entered on input80 to a linear equalizer 81. The linear equalizer is optimized for apredefined pulse as discussed above, and is shown in FIG. 7. A nonlinear equalizer 89 for reducing the inter symbol interference iscoupled to the linear equalizer 81 and provides an output signal to alinearizer 88. The non-linear equalizer comprises a number of delayelements 82,83,84 having a delay D of one symbol for determining aprevious and next signal at the previous and succeeding symbol readouttime. The previous and next signal are coupled to ISI calculators 85,86for calculating a correction value. The correction values are subtractedfrom the main signal in summing unit 87. The ISI calculator is based onthe model of the channel as discussed above, and is shown in FIG. 10.

FIG. 7 shows a linear equalizer. The implementation of the equalizer isas a discrete-time digital filter using the finite impulse response(FIR) structure. A read signal is received on an input 32 to a series ofdelay elements 33. The input signal is coupled to a first input of amultiplier unit 34, which has a filter coefficient 35 called C₀ on asecond input. The delayed versions of the input signal are coupled torespective multiplier units having corresponding coefficients (C₁, C₂,C₃, C₄). The results from the multiplying are coupled to a summing unit36 to generate an output signal 37. The coefficients of the linearequalizer are chosen such that the output result is free of ISI for theparticular selected value of mark length p. As explained above ISI-freeoperation for all p is impossible in pulse width modulated systems. Fora given storage system the channel response is known, and thecoefficients can be determined in advance. Alternatively, or as anadditional calibration, the coefficients are determined adaptively by anadaptive equalizer method.

FIG. 8 shows a linear equalizer and adaptation of the coefficients. Alinear equalizer unit 40 corresponds to the linear equalizer describedabove with reference to FIG. 7, wherein the coefficients 35 areadaptable based on the output of a least mean square unit 44, asindicated by arrow 45. The output 37 of the linear equalizer 40 iscoupled to a symbol detector 41, which detects the symbols received. Theoutput of the symbol detector 41 is coupled to a target response unit42, which provides the desired response signal after equalization forthe symbols detected. A summing unit 43 compares the desired responseand the real signal on the output 37, which provides the input signalfor the least mean square unit 44. The LMS equalizer aims atminimization of the mean square error between the output of theequalizer and a chosen target response of equalizer plus channel, whenused to equalize a channel that suffers from ISI and noise (see forexample the book; J. W. M. Bergmans: Digital baseband transmission andrecording, Kluwer, Boston, 1996 ISBN 0-7923-9775-4). The target responseis a full-response function (in discrete time: a delta impulse in caseof bit synchronous implementation). However it is to be noted, thataccording to the invention, the adaptation is restricted to a particularpulse length. For example, when the linear equalizer is set for a pulselength of 8, only symbols of that length are selected for use in thealgorithm. The update algorithm is arranged such that other pulselengths do not contribute to the adaptation.

FIG. 9 shows an alternative circuit for read signal equalization. Thelinear equalizer 81, the ISI calculator 85, the summing unit 87 and thelinearizer 88 correspond to FIG. 6. The non linear equalizer comprises adifferent number of delay elements, a first chain of delay elements91,92,93 delays the correction values of a single ISI calculator 85. Asecond chain of delay elements 94,95 delays the input signal. Thecorrection values are subtracted from the main signal in summing unit87.

FIG. 10 shows correction values for an ISI calculator. A curve 101 showsthe relation between input and output. A table 102 gives the numericalrelation from input to output. The correction values are based on thevalues shown in FIG. 5 for an equalizer at p=4.5 and the next neighborn=1.

FIG. 11 shows correction values for a linearizer. A curve 103 shows therelation between input and output. A table 104 gives the numericalrelation from input to output. The correction values are based on thevalues shown in FIG. 5 for an equalizer at p=4.5 and the nominal signaln=0. It is to be noted that the linearizer may also be combined with adetector/discriminator which receives the output value of the readsignal after equalization and converts the multilevel read signal in adigital value, e.g. a 3 bit value for each symbol.

It is noted that the equalizer described above is particularly suitablefor use in multilevel system in optical recording. However the system isalso suitable for other types of recording using different pulse shapes,wherein the equalizer can be optimized for one of the pulse shapes onlyand further pulses cause residual ISI. Also the system is suitable forread-only systems, because then no influence on the write channel isavailable and equalization can only be applied in the read channel.

In an embodiment the correction values established by the model asdiscussed above are augmented by read calibration. A record carrier maybe provided with known test patterns, which can be read and analyzed foradapting parameters in the equalizer. Also other learning patterns on adisc or signals detected from data may be used to adapt the equalizerparameters to the actual record carrier. For example in the read signalpredefined elements may be selectively used to calibrate the equalizers.Only read signals due to marks having the predefined length for whichthe linear equalizer is optimized are used to calibrate the linearequalizer.

It is noted that there is a relation between the selection of theequalizer and the residual ISI. When a specific selection of equalizeris predefined for the read channel, the optimization of a write strategypower pattern can be adapted to that equalizer. Hence the power patternsdefined for the different marks are adapted to a presumed read channeland equalizer. From the table in FIG. 5 it appears that if the equalizeris optimized for the longest pulse length, the main non-linear ISI andnonlinearity is associated with the pulses of moderate length. A writestrategy for pulses of moderate length usually allows more freedom foradaptation then the write strategy for the shortest or longest pulses.Hence preferably the equalizer is optimized for a longer, in particularthe longest, mark. The power pattern for writing the longest mark can beoptimized for maximum read signal, i.e. no additional requirements forreducing ISI are necessary at the write strategy because the readequalizer is optimized for such pulses.

Although the invention has been explained mainly by embodiments usingthe multilevel optical recording systems, the invention can be used forbinary recording systems also, e.g. for retrieving the location of thezero crossings of the read signal. It is noted that in this document theword recordable includes re-writable and recordable once. Also for theinformation carrier an optical disc has been described, but other media,such as optical card or magnetic tape, may be used. It is noted, that inthis document the word ‘comprising’ does not exclude the presence ofother elements or steps than those listed and the word ‘a’ or ‘an’preceding an element does not exclude the presence of a plurality ofsuch elements, that any reference signs do not limit the scope of theclaims, that the invention may be implemented by means of both hardwareand software, and that several ‘means’ may be represented by the sameitem of hardware. Further, the scope of the invention is not limited tothe embodiments, and the invention lies in each and every novel featureor combination of features described above.

1. Device for scanning a track (11) on a record carrier (4) for readinginformation, the device comprising: a head (22) for generating a readsignal via a beam of radiation for scanning marks in the track, themarks having a number of different shapes for representing theinformation, read means (30) for processing the read signal, the readmeans comprising a combination of a linear equalizer (81) and anon-linear equalizer (89) for equalizing the read signal, the linearequalizer (81) being arranged for equalizing based on a mark having asingle predefined shape, and the non-linear equalizer (89) beingarranged for reducing inter symbol interference in the read signal formarks having a different shape then said predefined shape, the intersymbol interference remaining in the read signal due to said linearequalizer being based on said single predefined shape.
 2. Device asclaimed in claim 1, wherein the read means (30) are for processing theread signal for generating a corresponding number of different levels ofa processed read signal at a read-out time for said number of differentshapes.
 3. Device as claimed in claim 1, wherein said number ofdifferent shapes comprises longer and shorter shapes, and the linearequalizer (81) is arranged for equalizing based on a mark having alonger shape.
 4. Device as claimed in claim 3, wherein the linearequalizer (81) is arranged for equalizing based on a mark having thelongest shape of said different shapes.
 5. Device as claimed in claim 1,wherein the linear equalizer (81) comprises a finite impulse responsefilter having delay elements coupled to multiplying elements, themultiplying elements containing multiplying coefficients that are basedon said single predefined shape.
 6. Device as claimed in claim 1,wherein the non-linear equalizer (89) comprises delay elements coupledto an inter symbol interference calculator (85,86), the inter symbolinterference calculator being arranged to calculate the inter symbolinterference based on a sequence of read signal values representing asequence of marks.
 7. Method of equalizing a read signal during readinginformation in a track on a record carrier, the method comprisingreceiving a read signal generated by marks in the track, the markshaving a number of different shapes for representing the information,processing the read signal by a combination of linear equalization and anon-linear equalization for equalizing the read signal, the linearequalization being arranged for equalizing based on a mark having asingle predefined shape, and the non-linear equalization being arrangedfor reducing inter symbol interference in the read signal for markshaving a different shape then said predefined shape, the inter symbolinterference remaining in the read signal due to said linear equalizerbeing based on said single predefined shape.